Polyurethanes are formed by the reaction of polyols and isocyanates with an average of 2 or more functional groups per monomeric unit. The length of the segments between functional groups and the number of functional groups determine if the resulting polymer is flexible or rigid and the suitability of the polymer for making foam. These parameters also determine if the polymer is considered to be thermoset or thermoplastic. Chain extenders, crosslinkers, catalysts, and surfactants can be added to change these properties. Polyurethane applications can be classified as solid or foamed products with the latter being subdivided into foams that have different densities and different cell structures.
Nanocrystalline cellulose (NCC) is crystalline particle that have a high intrinsic strength, nanometric dimensions and a surface that is populated with hydroxyl groups; NCC are in essence nanoparticulate polyols. One example of cellulose nanocrystals is a sulphated cellulosic particle produced by sulphuric acid digestion of cellulose and being manufactured as CelluForce NCC™. The particles produced in this way are uniform and have an aspect ratio of about 20; the exact length and diameter of the particles are dependent upon the starting substrate and the process conditions that are used.